Atomic layer deposition (ALD) is a process for depositing highly uniform and conformal thin films by alternating exposures of a surface to vapors of two chemical reactants. For example, it is useful for depositing high-permittivity metal oxide dielectrics such as ZrO2 or HfO2 to replace SiO2 in future generation metal-oxide-silicon (MOS) transistors. Other applications include luminescent materials for photonics, thin film phosphors for electroluminescent displays, and catalysts for fuel cells. A prior art ALD process 100 is shown in the schematic drawing of FIG. 1. It consists of an alternating series of self-limiting chemical reactions, called half-reactions, between gas phase precursors and the substrate. The precursors are pulsed into the reactor in a sequential fashion. Between each half-reaction, an inert gas flow is used to purge the growth chamber of the previous precursor species. ALD processes are typically performed at modest temperatures (300-600° K). The material grows at a rate of up to one monolayer at a time, and depositions of ˜1 Å per cycle, and cycle times of 1-100 sec. are typical. Consequently, it is most useful for deposition of film structures with a total thickness of ˜50 nm or less.
Nanomaterials such as nanoparticles are of interest for many applications, including as optical tags and sensors, as lasers, in photovoltaics, in molecular electronics, in photonics, and as catalysts. For many applications, it is not just the synthesis, but the assembly or distribution of the nanoparticles or nanomaterials that is important. For example, for heterogeneous catalysis it is desirable that the nanoparticles be dispersed throughout a high surface area support to have a high number of active sites. In fuel cell applications, dispersed nanoparticle catalysts or catalysts distributed in a mesh structure will allow reduction of the noble metal loading (reduced cost), and increase in the triple-phase boundary (increased efficiency). Nanoparticle dispersion can also lead to high sensitivity in sensor applications. Similarly, for metal nanoparticles used as the catalysts in nanotube or nanowire growth, distributing the nanoparticles spatially on a planar substrate can lead to assembled and oriented nanotubes or nanowires. Although there are many techniques available for synthesizing nanoparticles and other nanoscale objects, there is still an important need for methods that control both the particle size and the distribution of the nanoparticles in two or three dimensions. In addition, techniques that can deposit nanoparticles and other nanoscale objects uniformly throughout a porous substrate are key to many applications in catalysis and sensing.
Nanoparticles have been synthesized by a variety of methods. The most common method of producing them makes use of colloidal chemistry. Metal nanoparticles can be produced in solution by the reduction of a suitable salt in the presence of a stabilizer. Another preparation method is the thermal decomposition of organometallic precursor of various metals in the presence of suitable surfactants. Additionally, vaporization of the material followed by deposition onto a cold substrate or inside walls of vacuum chamber has been successfully carried out. A related method is the laser ablation technique in which nanoparticles are generated by irradiation of a solid target by a focused ion beam under inert conditions and then deposited on cold substrates. A number of gas phase deposition techniques have been employed as well. Semiconductor nanoparticles have been synthesized in large quantities using these methods. Furthermore, anatase titania nanoparticles have been prepared by metalorganic chemical vapor deposition. All of the aforementioned techniques have lacked control of the size and spacing of the desired nanoparticles.
Accordingly, there is a need to develop method for growing nanoparticles on both flat and structured surfaces in which the size and average spacing of the nanoparticles can be controlled.